Rise in dormancy levels calls for strict strategies

Germination of blackgrass and ryegrass is likely to be more protracted this autumn and growers will have to adapt control strategies from the word go, latest research reveals.

Blackgrass has shown the highest dormancy, contrasting sharply with the past two years when dormancy has been much lower. ADAS testing of 41 geographical samples found that only 25% germinated within two weeks of collection this year, compared with 53% in 2006.

“Blackgrass seeds this year are more dormant and unwilling to germinate, even with adequate seed-bed moisture,” says ADAS Boxworth’s Sarah Cook, who led the HGCA-funded project. “This is unlike the last two years where seeds had low dormancy and germinated readily when moisture was available.” Similar high dormancy was seen in 2002 and 2004.

Work at Rothamsted Research by PhD student Rocio Alarcon-Reverte (funded by HGCA and Syngenta in association with University of Reading) found a similar pattern of higher dormancy in 34 ryegrass samples, where on average 53% germinated, compared with 72% last year. There is some variation for both weeds, highlighting the importance of local conditions – for example ryegrass germination ranged from 26 to 97%.

Cool, wet and dull conditions during seed maturation in June and July are thought to be behind the higher dormancy, reckons Rothamsted’s Stephen Moss, who says there will be implications for pre-sowing cultural controls and autumn herbicides.

Ryegrass tends to shed its seed later than blackgrass, so the window for tackling plants that do germinate pre-sowing with glyphosate can be tight, especially before early cereal drillings and oilseed rape, he explains.

So, with less of the weed likely to be vulnerable in that slot this season, he says residual pre-emergence treatments could be valuable. “Pre-emergence herbicides, such as Defy [prosulfocarb] and Crystal [flufenacet + pendimethalin] can be useful at controlling ryegrass, especially if it emerges over a long period.”

In fields with high blackgrass populations, Dr Cook says burying seeds by ploughing stubbles can be useful in high dormancy years, although growers who achieved good control last year must be careful not to plough up more seeds than are buried.

She encourages growers to spray off blackgrass that emerges before drilling and says early drilling may be appropriate to establish a competitive crop. “Do not be tempted to reduce seed rates where high populations of blackgrass are expected.”

Dr Cook also recommends using a robust pre-emergence herbicide with a residual component to cover protracted germination.

ADAS colleague James Clarke says in high dormancy situations, blackgrass germination can be 6-8 weeks later than low dormancy years and consequently, stale seedbeds will be less effective. “It’s still worth spraying off what germinates now, but a lot of blackgrass could come up within the life of the crop, so you’ve got to hit them hard with the pre-em. When you get to the post-em, it is likely there will be more to come, so you need that residual element as well.”